20 seaweed are a possible prize from Treasure Hunter[source needed]. When choosing the option to convert this prize to coins, players receive 500 coins during the Double Cash period, implying that normally they would be converted to 250 coins, which is 1,230 less than selling them on the Grand Exchange.

Cooking seaweed into soda ash yields a profit of 4 coins each.

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Seaweed can be found in several places, along shorelines and sometimes rivers. Some locations are:

Karamja. The western and eastern shores of the central part of the island have seaweed, as does the northern shore of east Karamja (south of Musa Point). North bank of the river that flows through Shilo Village has 3 seaweed.

North eastern Entrana. 15 total: 9 east of the chicken pen, 6 north of the Law Altar. (Weapons and armour cannot be taken to Entrana.)

Northern Fremennik Province, in the Rock Crab area north-west of the entrance to Keldagrim. This area has 9 seaweed spawns and is recommended. With a full inventory of seaweed nets, one can make a rather large profit. The area is accessible by using the cave to go to Keldagrim and using the bank there, or by using Fairy rings allows fairly quick access between this area (coordinate D•K•S) and the bank at Zanaris. (Players must complete at least part of the quest A Fairy Tale Part II to have full access to the fairy rings.) Players can use the Rellekka lodestone to teleport fairly close.

Although not technically free, the purchase price in the section below is so low that it might as well be free, considering the Grand Exchange price.

Northern Fremennik Province. Log onto a high volume world and bring 27 seaweed nets to Northern Fremennik Province to the Rock Crab area north-west of the entrance to Keldagrim. In about 15 minutes, you can get 27 full (10) seaweed nets. Potentially dangerous due to level 102 Rock Crabs. Use Fairy RingD•K•S or A•J•R or Lodestone teleport.

Seaweed can be bought from Arhein in Catherby for 2 coins each. Arhein does not stock the seaweed in his shop; instead, players must choose to talk to Arhein and use the prompt: "Can I buy some seaweed?". Arhein will only sell 80 seaweed per day. He sells the seaweed noted. There is both a range and a bank near Arhein, which is quite convenient for rapidly turning the seaweed into Soda ash.

Additionally, players may be able to purchase seaweed on the Grand Exchange, or by trading with another player.

Seaweed is very commonly used for making money. Examples would include:

Buying 80 Seaweed a day at Catherby from Arhein for 2gp each and selling them in the G.E. for a total profit of 5,760. This will usually sell quickly on the Grand Exchange.

Buying Seaweed from the Grand Exchange, cooking it into soda ash and selling it yields a profit of 4 coins each.

Catching Seaweed using a Big fishing net in certain fishing spots around RuneScape that are close to a bank, such as Catherby.

After completing Deadliest Catch, Fisherman Jones will reward you with several deposit chests; one of which is located south of the Karamja lodestone. If walking isn't an issue this chest can be used to collect seaweed for an indefinite period of time.